Patent Application
20110020574
a. Field of the Invention
The present invention relates to extrusion-processable materials having a plurality of capillary channels therethrough, and to apparatus and methods for manufacturing such materials. The capillaries may be occupied by gases, liquids or other phases. The materials may be anisotropic foams which have high voidage.
b. Related Art
Conventional polymer foam is usually extrusion processed, for example as described in Tuladhar, T R and Mackley, M R, “Experimental observations and modelling relating to foaming and bubble growth from pentane loaded polystyrene melts”, Chemical Engineering Science, vol. 59 (24), 5997-6014, 2004. The resultant product contains a random, isotropic voidage that is inherently difficult to control and predict.
Thermoplastic films or ribbons with anisotropic parallel multiple capillary channels running through them are known as microcapillary films (“MCF”). MCFs and their manufacture are described in WO 2005/056272. MCFs typically have about 5-6% voidage based on optical analysis and the geometry of the die and air entrainment device.
It is desirable to make MCFs having a much higher voidage, to provide anisotropic foam-like structures having parallel channels. Such materials provide interesting mechanical properties and the possibility of new applications. Previous attempts to produce high-voidage MCFs by upstream gas injection have failed because of blistering of the capillaries (Hallmark B, The Development of Microcapillary Films, PhD Thesis, Chapter 3, University of Cambridge, UK, 2005.)
According to an aspect of the invention there is provided a method of producing an extrudate product having a plurality of capillary channels therethrough, the method comprising the steps of:
By quickly quench cooling the extrudate, we have found that stable MCFs with high voidage may be made. Product with voidage up to about 30% can be made using entrainment of air or other fluid at ambient pressure, and we have found that extruded product with voidage in excess of about 30% can readily be produced by injecting pressurised fluid via the injectors. The process overcomes problems of blistering of the capillaries when using a source of pressurised fluid. Using the process we have found that anisotropic thermoplastic foam materials may be controllably formed with voidage up to 60%, and we believe it will be possible to produce anisotropic extrudate product having even higher voidage of up to 80 or 90%.
It will be understood that ‘quench cooling of the extrudate as it is exits the die’ is to be interpreted practically, as in practice there will always be a finite distance between the die exit and the point at which the extrudate has been sufficiently cooled to resist mechanical deformation. The term ‘die exit’ means the first location at which the external boundaries of the extrudate are no longer constrained by the die. It is preferred that the cooling is applied to the extrudate less than about 3 mm from the die exit, preferably less than 2 mm, and particularly preferably about 1 mm or less. Thus, it is preferred that the melt drawing length (distance from the die exit to the point of quenching) is as close to zero as practically possible. In one embodiment the quench cooling is provided by one or more fluid jets, notably high speed air jets, which can be directed at the extrudate close to the die exit. It would also be possible, in another embodiment, for quench cooling to be provided by cryogenic immersion.
In one embodiment of the invention, the fluid source is a regulated, compressed gas supply, notably a pressurised air supply, which can provide a flow of gas under pressure to the fluid injectors. The fluid injectors comprise hollow bodies through which fluid can pass, for example needles. Under conventional MCF extrusion conditions the injectors are passive devices that entrain air naturally at atmospheric pressure. The effect of using pressurised gas is to inflate the capillaries.
The quenched film is typically taken through a pair of nip rollers and then spooled. We have found that the pressure of the nip in this process may be high enough to destroy capillaries formed in a conventional MCF process.
Potential applications for the extrudate product include capillary electrophoresis, for example for DNA analysis, heat transfer, fluid transport, micro-reactor, biomedical applications, liquid encapsulation, optical applications, foodstuffs and porous matrix applications. Such applications are described in WO 2005/056272 and U.S. Ser. No. 10/582,465, the entire contents and disclosure of which are incorporated herein by reference. Other non-limiting examples of applications for the anisotropic foam product include use as food, drug or gas conduits, for example drinking straws, in-body applications, or as pneumatic or hydraulic actuators with ‘spark-safe’ movement. Further potential applications include remote sensing applications such as conduits for the detection of chemicals, drugs or pollutants, and pressure conduits for measuring pressures at different points in a system or vehicle. The product when used in the applications may have multiple channels or it may be severed to produce a plurality of single channel products.
An important consequence of the ability to fabricate anisotropic foam ribbons is the ability to make an anisotropic foam monolith by heat-melding together a stack of anisotropic foam ribbons. In an alternative embodiment, a two-dimensional array of injectors may be used to create a foam monolith by direct extrusion. Such a material is expected to have unique mechanical properties which can be tailored to suit particular applications.
It is preferred that the die orifice is substantially rectangular so the resulting outer shape of the extrudate product is substantially rectangular. The dimensions of the rectangular orifice are preferably such that the extrudate product is a sheet, film. or ribbon Preferably the rectangular orifice has a long side having a length that is at least 5 times longer than the short side. Preferably the ratio is greater than 10 as this may allow the film to flex more readily. It should be understood that the orifice could take any other suitable shape, including an annulus, square or circle. It has been noted that with a non-circular die, for instance a rectangular die there may be edge effects that alter the shape of the capillaries at or near an edge of the film. Such edge effect may be negated through the use of an annular die which is, in effect, a continuous film having no edges. An annular die may allow the production of a tubular extrudate product, for example a cylindrical tube, having greater consistency in the size and shape of the capillaries.
By quench cooling the extrudate as it exits the die, the cross-sectional shape of the extrudate product may be fixed in substantially the same shape as the internal shape of the die orifice. For example, the extrudate product may comprise a tube of circular, oval, square, rectangular, triangular or other polygonal cross-sectional shape, having parallel bores or channels within the wall(s) of the tube.
For simplicity the apparatus will now be described with reference to the preferred embodiment in which the die has a substantially rectangular orifice in which an array of needle outlets are arranged in a line substantially parallel with the long side of the rectangle and substantially in the centre of the short sides of the orifice. This produces an extruded film having a plurality of bores or capillary channels therealong. It should be understood that different arrays and orifice shapes could be employed.
The invention will now be further described, by way of example only, with reference to the following drawings in which:
The melt 24 passes to the die 14 through an extruder barrel 20 which is connected to the gear pump by a flange 22. Band heaters 26 are used to control the temperature at different stages in the extrusion apparatus 1. Band heaters 26 may be located within the extruder, on the flanges 16,22, on the gear pump 12, on the extruder barrel 20 and also on the die 14. The detail of exemplary embodiments of the die 14 will be shown in greater detail in subsequent figures.
The melt passes through the die 14 and is formed into the desired shape and cross section. As the melt passes out of the die it becomes an extrudate 28 (
The die 14 further includes injectors, which in this example comprise needles 38 (only one of which is shown in this figure) positioned therein. Each needle 38 has a body portion 40 having a conduit 42 therein which is fluidly connected to a fluid source 44 by means of a second conduit 43 passing through a wall of the die 14 around which the melt must flow to pass to the orifice 36. The needle 38 further includes an outlet 46 at an end 48 of the needle 38. The needle 38 is arranged such that the outlet 46 is located within the orifice 36. In this embodiment, the fluid source 44 is a source of compressed air. However other fluid sources may be used. The fluid source could for example comprise a different compressed gas, a liquid, sol or suspension, or the fluid source 44 could be open to atmosphere to allow ambient air to be entrained.
The die 14 is also designed so that the flow over the array of needles 38 is substantially even. An even melt flow around the needles 38 facilitates creation of well-formed extrudate 28. If, however, there is an uneven flow, the melt will preferentially channel along a path of least resistance. This results in a distorted extrudate 28, which can also results in inconsistent draw down distortions.
In this example, the die includes ten needles 38 with the outlets 46 distributed substantially evenly along the long axis 53 within the orifice and substantially centrally in the orifice along the short axis 51. In this example, the die orifice has a short side dimension of 1.5 mm, a long side dimension of 18 mm and the needles have a 0.5 mm outer diameter and a 0.3 mm inner bore.
In this example, compressed air from the fluid source 44 is provided to the second conduit 43 via a supply conduit 3 with a manually-controlled isolation valve 5. The second conduit 43 has a mass flow control valve which can be used to regulate the volume of gas passing through the needles 38. A pressure sensor P is used to measure the pressure of gas to the needles 38.
The supply conduit 3 is fluidly connected to air jet lines 15 via a manually-controlled needle valve 7. The air jet lines 15 are arranged to direct jets of high-speed air via air-jet nozzles 11 to extrudate 28 as it exits the die orifice 36, thereby rapidly quenching the extrudate 28 to form the extrudate product 2. A temperature sensor T and a pressure sensor P can be used to measure temperature and pressure of the air jet lines 15.
In the embodiment of die 14 shown in
The web of quenched extrudate product 2 is held by a pair of nip rollers 30, which can be pressurised so as to get a firm grip. This permits the nip rollers 30 to maintain a good hauling tension. Because the nip rollers 30 are not handling readily-deformable extrudate 28, they may use pressures much higher than in a conventional MCF process. The nip rollers 30 may be chilled, but this is not essential since rapid air-quenching has taken place close to the die exit. After passing through the nip rollers 30 the extrudate product is wound on to a spool 31, in this example via one or more guide rollers 29, either or any of which may optionally by heated.
A key difference between the process of the present invention and prior art MCF processes is illustrated in
The process enables the controlled production of high-voidage extrudate products, including novel anisotropic foam materials.
The apparatus of
Experiments were carried out using a commercial grade of linear low-density polyethylene (LLDPE NG5056G) manufactured by the Dow Chemical Company Inc. Prior to starting experimental work, the extruder was heated to 175° C. and extrusion line (the remaining ancillary pipe-work, pumps and die) to 165° C. Once at these temperatures, the extruder was set to run at a volumetric flow-rate of approximately 2×10−7 m3/s. This corresponded to a die exit velocity of roughly 7 mm/s.
At all times during the experiments, the high-speed gas flow was used to quench the polymer at the moment it exited the extrusion die. The two gas quenching nozzles 11 were situated 10 mm above and below the die exit, lined up on the die exit's centreline. These quenching jets are visible in
Seven sets of experiments were carried out to determine the effects of specific process parameters on the product voidage; these parameters were the die temperature, hauling speed and gas flow to the injectors. The details of each set of experiments are given in Table 1.
Depending on the stability of the product, around 6 to 8 product samples were taken for each set of experiments at differing increments of the process parameter under investigation.
Each sample was then sectioned with a razor blade, exposing the capillary structure, and viewed using a CCD camera with a high-magnification lens. The images of each cross section were captured on a PC and then analysed to give an estimate of the voidage present within the film.
The effect of increasing the gas flow to the injectors 11 had two effects on the extruded product 2. Firstly, between gas flows of 0 cm3/min (natural entrainment) and about 300 cm3/min the process was stable, producing products of increasing voidage. These results are summarised in Table 2.
Photographs of sections of the extrudate product 2 for Experiments A1-A3 are shown in
If the flow-rate had been incrementally increased from zero, the process went from being stable to unstable at flow-rates above 340 cm3/min. The instability manifested itself in the form of internal rupture of the product, shown in
It was found, however, that flow-rates between 340 cm3/min and 500 cm3/min could be stably sustained (on occasion) if they were imposed rapidly from a zero flow rate. It is thus preferred to take the process from zero flow rate to the intended maximum flow rate as quickly as possible, notably in less than 5 seconds, preferably in 2 seconds or less. For a 19 capillary product, the preferred flow rate is therefore in the range 0 to about 26 cm3/min (500/19).
The effect of increasing the haul-off speed on the product was seen qualitatively to increase the voidage within the product. Quantitative measurements are not presented here since the haul roller pressure was too great for the films and consequently some of the capillaries had been significantly reduced in size hence invalidating comparative voidage measurements.
Several other phenomena, however, were noticed during the course of these experiments. The first was that increasing the haul-off speed did not have a strong effect on the external dimensions of the product. Usually, increasing the hauling rate reduces the dimensions of the film as it is drawn in its molten state between the die exit and the quenching roller. Due to the immediate quench at the die exit, however, this melt drawing did not occur in the same manner. The net effect of increasing the haul-off speed is thought to increase the drawing force of the polymer at the die exit.
It was noticed, however, that the pressure of the polymer melt (measured significantly upstream of the extrusion die), was affected by the haul off speed, such that increasing the drawing speed decreased the melt pressure. This effect has not been previously observed with the conventional MCF process. Without wishing to be bound by theory, a possible explanation for this effect is that the rapid gas quench causes a large temperature gradient in the polymer at the die exit, resulting in locally higher polymer viscosity in this region when compared to the viscosity of the polymer within the extrusion line. Increasing the haul-off speed, therefore, has the net effect of being able to forcibly remove more material from the die exit region, resulting in an acceleration of the polymer within the extrusion die. Typically, the majority of the pressure drop within the extrusion line occurs around the region of the die exit, so if material is being removed from this area by a means other than the pressure within the extrusion line, then the measured pressure within the extrusion line will fall.
A further observation was with regard to product quality in relation to haul-off speed. At low haul-off speeds (0.5 m/min), a high-quality finish was achieved on the polymer. At higher hauling speeds (around 0.8 m/min), however, the quality of the finish became very rough and ‘ragged’. This effect is illustrated in
These experiments explored the effect of haul-off speed and injector gas flow on the final product. For these three experiments, the die exit temperature was raised by 20° C. to 185° C. The purpose of this was to delay the onset of the ‘ragged’ type of product illustrated in
The effect of both increasing injector gas flow and increasing haul-off speed was to increase the voidage in the final product. This is summarised in Table 3.
In addition to the increasing voidage, as a function of both gas flow and haul-off speed, some other interesting observations can be made from these data. Firstly, the process remained stable throughout in relation to maintaining un-ruptured capillaries. Secondly, it can be seen from the data in the table that increasing haul-off speed decreased the extrusion pressure, as observed in experiments B and C. Lastly, again similar to experiments B and C, increasing haul-off speed only had a minor effect on the external dimensions of the films. A plot showing the variation in film width and film thickness with haul-off speed is shown in
The final experiment examined the effect of haul-off speed on voidage and product quality at an elevated die temperature of 215° C. This was carried out under conditions of natural air entrainment through the injectors 38.
As expected from previous results, it was found that increasing haul-off speed increased the amount of voidage present within the film; two examples are given in Table 4.
The most striking observation from this set of experiments, however, concerned the stability of the process in terms of draw resonance.
Draw resonance is an instability found in fibre-spinning and film-casting processes. If we define a ‘draw-ratio’ as the ratio of (mean) velocities between the polymer coming out of the die and the velocity with which it is hauled away, post quench, in its solid state, then we can see that this quantity related to the amount of draw-down the polymer experiences in its molten state.
If one attempts to try and draw the polymer excessively, then the ‘draw-resonance’ instability appears. It takes the form of periodic oscillations in the width and thickness of the film. The period of the oscillation is linked to the quenching distance (distance between die exit and quench) and the amplitude linked to the draw ratio. It can be proved mathematically that, for a Newtonian fluid, draw resonance will appear at a draw ratio of 20.2. The are various ways in which viscoelasticity, fast quenching, and other methods can be combined to slightly delay the onset of this instability. In general, however, it represents the upper production limit on the film casting/fibre spinning system.
At no point was draw resonance observed. At the highest haul-off speed attainable with the existing machinery (15.2 m/min), the process was stable. It was, however, observed that due to the cooling effect of the quenching jets, the velocity of the polymer exiting the die appeared to be substantially similar to that of the haul-off speed. It furthermore appeared that the polymer was being forcibly pulled out of the extrusion die exit, with a consequent acceleration of the polymer within the extrusion die and drop in pressure in the extrusion line.
The elevated die temperature eliminated any observable form of product ‘raggedness’ that has previously been discussed. Furthermore, due to the lower viscosity of the polymer within the die, there was some drawing effect in terms of the external dimensions of the film diminishing with increasing hauling speed; this effect was still significantly weaker when compared to similar effects in the conventional MCF process.
A photograph of the cross section of the film from experiment G5 also demonstrates the higher haul-off speeds being used resulted in a shape- and size-range of capillaries; this is shown in
Some experimental work carried out using a polyolefin plastomer (Dow Affinity 1880) has suggested that variation of the process parameters outlined using LLDPE have similar effects. A key difference is that is appears to be easier to produce higher voidages but also easier to induce the rupturing instability that was shown in
Applications for these new high-voidage products, including anisotropic foams, include novel conduits for foods, drugs or gas, such as drinking straws, which may have multiple capillaries. A particularly useful feature is their ability to sustain curvature in one direction without collapse of hollow capillaries.
There are a number of factors that influence the voidage of the final product. These can be summarised as follows:
In the sets of experiments with LLDPE, voidages between 17% and up to around 35% were observed. In experiments with Affinity plastomer, voidages up to 60% were observed. Successful experiments have also been carried out with an extrusion grade of poly(vinyl alcohol) (PVA) (Depart PVA C10-40).
In addition to these observations, some unusual process characteristics also emerged. These can be summarised as follows:
By adjusting the process conditions, an extrudate product can be formed with a high degree of voidage. The voidage may be filled by a second phase such as air, or by any other type of phase. The surface area to volume ratio can be ‘tuned’ for specific applications, and a product can be made which is tough and can take tensile loading, and which will bear a curvature without fracturing which would damage a single-bore tube.
The articles ‘a’, ‘an’ and ‘the’ are used herein to denote ‘at least one’ unless the context otherwise requires.
It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately, or in any suitable combination.
It is to be recognized that various alterations, modifications, and/or additions may be introduced into the constructions and arrangements of parts described above without departing from the ambit of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0620246.9 | Oct 2006 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2007/003010 | 10/9/2007 | WO | 00 | 3/11/2010 |
